3.4.2.1 Bulk Properties of Solids

Question 1

What is density?

Answer 1

Density is mass per unit volume. It is a measure of the compactness of a substance.

Question 2

What is the equation for density?

Answer 2

Density = mass/volume

Question 3

What is unit for density?

Answer 3

Density = g/cm^3 or kg/m^3.

1 g/cm3 = 1000kg/m3

Question 4

What does the average density of an object determine?

Answer 4

The average density determines whether it floats or sinks.

An object will float on a fluid if it has a lower density than the fluid.

Question 5

What does Hooke’s Law state?

Answer 5

Hooke’s law says that the extension is proportional to force.

Question 6

What happens to a metal wire if a load is attached?

Answer 6

If a metal wire is supported at the top and then a weight is attached to the bottom, the wire stretches. The weight pulls down with force F, producing an equal and opposite force at the support.

Question 7

What is Hooke’s law equation?

Answer 7

Force = stiffness constant x change in extension.

Question 8

What do tensile forces do to a string?

Answer 8

They stretch the spring.

Question 9

What do compressive forces do to a spring?

Answer 9

They squash a spring.

Question 10

What is the limit of proportionality?

Answer 10

The limit of proportionality is the point of which a wire stops obeying Hooke’s law.

Question 11

What is the elastic limit?

Answer 11

The point of which if the force is increased, the material will be permanently stretched. When all the force is removed, the material will be longer than at the start.

Question 12

What is elastic deformation?

Answer 12

If deformation is elastic, the material returns to its original shape and size once the forces are removed. It happens as long as the elastic limit of the material is not reached.

Question 13

Explain elastic deformation in terms on atoms?

Answer 13

When the material is put under tension, the atoms of the material are palled apart from one another.

Atoms can move small distances relative to their equilibrium positions, without actually changing position in the material.

Once the load is removed, the atoms return to their equilibrium distance apart.

Question 14

What is plastic deformation?

Answer 14

If deformation is plastic, the material is permanently stretched. An object stretched past its elastic limit shows plastic deformation.

Question 15

Explain plastic deformation in terms of atoms.

Answer 15

Some atoms in the material move position relative to one another.

When the load is removed, the atoms don’t return to their original positions.

Question 16

What is conserved when stretching?

Answer 16

Energy is always conserved.

Question 17

Where is the energy stored in elastic deformation?

Answer 17

If a deformation is elastic, all the work done is stored as elastic strain energy in the material.

Question 18

What happens to the energy once the force is removed?

Answer 18

When the stretching force is removed, this stored energy is transferred to other forms.

Question 19

Where is the energy stored when deformation is plastic?

Answer 19

If a deformation is plastic, work is done to separate atoms, and energy is not stored as strain energy and is mostly dissipated as heat.

Question 20

Explain how vehicles are designed for safety with use of elastic properties.

Answer 20

Crumple zones are designed to deform plastically in a crash. Some energy goes into changing the shape of the vehicle’s metal body and so less is transferred to the people inside.

Question 21

What is tensile stress? Give the equation and units.

Answer 21

Tensile stress is defined as the force applied, divided by the cross sectional area.

Stress = Force / Area.

Unit = N/m2 or pascals.

Question 22

What is tensile strain? Give the equation and units.

Answer 22

Tensile strain is defined as the change in length divided by the original length.

Tensile strain = change in length (extension) / original length.

Strain has no units, it is just a ratio and is written as a decimal or percentage.

Question 23

What is the Breaking Stress?

Answer 23

A stress big enough to break the material is called the breaking stress.

Question 24

What happens as the tensile force applied to a material increases?

Answer 24

The stress on it increases.

Question 25

Explain the effect of breaking stress, in terms of atoms.

Answer 25

The effect of the stress is to start to pull the atoms apart from one another.

Eventually, the stress becomes so great that atoms separate completely, and the material breaks.

Question 26

What is UTS?

Answer 26

UTS is called the ultimate tensile stress. This is the maximum stress that the material can withstand.

Question 27

What does the area under a force-extension graph represent?

Answer 27

Elastic strain energy.

Question 28

Explain stretching in terms of energy.

Answer 28

Work has to be done to stretch a material.
Before the elastic limit is reached, all this work done in stretching is stored as elastic strain energy in the material.

Question 29

How can elastic strain energy be calculated?

Answer 29

Providing the material obeys Hooke’s law, the potential energy stored inside it can be calculated using the following equation:

Elastic Strain Energy = 1/2 x Force x Extension

or can be calculated using,

Elastic Strain Energy = 1/2 x Stiffness Constant x Extension^2.

Question 30

What two types of energy can spring energy be transferred to?

Answer 30

Kinetic and gravitational potential.

Question 31

What does the stress-strain graph for a brittle material look like?

Answer 31

Doesn’t curve.
It starts of as a straight line through the origin.
When the stress reaches a certain point, the material snaps, it doesn’t deform plastically.

Question 32

What does the force-extension graph of brittle materials look like?

Answer 32

Looks the same as stress-strain graph. No plastic deformation, so the line is straight until the material reaches the point at which it fractures.

Question 33

What are three main parts of a stress-strain graph?

Answer 33

Limit of proportionality.
Elastic Limit
Yield Point’

Question 34

What is the limit of proportionality on a stress-strain graph?

Answer 34

Symbol P.

After this point, the graph starts to bend.

At this point, the material stops obeying Hooke’s law but would still return to its original size and shape if the stress was removed.

Question 35

What is the elastic limit on a stress-strain graph?

Answer 35

Symbol E.
At this point the material starts to behave plastically. From this point, the material would no longer return to its original size and shape once the stress was removed.

Question 36

What is the yield point on a stress-strain graph?

Answer 36

Symbol Y.
Here the material suddenly starts to stretch without any extra load.

The yield point or yield stress is the stress at which a large amount of plastic deformation takes place with a constant or reduced load.

Question 37

What does the area under a stress-strain graph?

Answer 37

The area represented the energy stored in the material per unit volume.

Question 38

What happens when the load is removed from a material?

Answer 38

The extension decreases.

Question 39

Why doesn’t the unloading line go through the origin?

Answer 39

Because the wire was stretched beyond its elastic limit and deformed plastically, it has been permanently stretched. Therefore its extension will not go back to 0.

Question 40

What does the area between the loading and unloading line represent?

Answer 40

The area between the two lines is the work done to permanently deform the wire.